This invention relates to thermally actuated devices with accurate temperature response.
Thermally actuated devices which comprise a shape memory alloy (SMA) are well known. An excellent review about the industrial applications of the SMA material is given by C. W. Wayman, Journal of Metals, June, 1980. Among many SMA materials, Ti-Ni base alloys (which include Ti-Ni alloys according to U.S. Pat. No. 3,174,851, Ti-Ni-Co alloys and Ti-Ni-Fe alloys according to U.S. Pat. No. 3,558,369, and Ti-Ni-Cu alloy according to U.S. Pat. No. 4,144,057) are most practical.
The SMA material converts heat energy into mechanical energy directly. Two kinds of mechanical action "one-way" and "two-way" are known. One-way action involves that a shape change occurs only on heating. Two-way action involves that the shape change occurs both on heating and cooling.
In general, thermally actuated devices operable reciprocately are considered requiring the use of the SMA material capable of exhibiting the two-way action. The Ti-Ni system generally has a property of exhibiting the one-way action, but when combined with a bias load, it exhibits the two-way action. The general method for causing the two-way action of Ti-Ni base alloy is to:
(1) form an SMA helical coil of cold-drawn SMA wire;
(2) constrain the SMA helical coil as close state;
(3) anneal the SMA helical coil at about 500.degree. C. (which is called memory anneal or imprinting anneal);
(4) cool the SMA helical coil to room temperature;
(5) remove the constraint; and
(6) hang a bias load from the lower end of the SMA helical coil. The bias load means dead weight, bias spring, or other forces added against shape recovery direction.
On cooling below the transformation temperature range, the bias load produces a greater deflection of the SMA helical coil. On heating above the transformation temperature range, the SMA helical coil will contract to its imprinting close-coiled state. Therefore, we can get two-way action of the SMA helical coil.
In this two-way action, the transformation temperature range during the cooling and that during the heating do not match with each other, and the former is generally lower than the latter. This difference in transformation temperature range is called a temperature hysteresis.
According to the prior art, the temperature hysteresis has been 10.degree. to 30.degree. C. In addition, it has been found that, when the two-way action is repeated, the transformation temperature range tends to shift and deflection tends to increase, thereby reducing the recurring lifetime. These inferior properties have proven to be a major inhibiting factor in the development of a thermally actuated device comprising the SMA material with accurate temperature response.